The present invention relates generally to methods and apparatus for taking core samples, and more specifically relates to new and improved methods and apparatus to cut cores and to retain the cores while minimizing breakup or fluid deterioration of the cores.
Formation coring is a well-known process in the oil and gas industry. In conventional coring operations, a coring bit, adapted to cut a cylindrical core from the formation, is coupled to a core barrel assembly adapted to receive and retain the core. As the core is cut, it will traverse an inner gage-cutting portion of the bit to eventually reach a core shoe which accepts the core and guides it into an inner retention tube.
Some significant problems are encountered when coring is performed in relatively soft to medium hard, or unconsolidated formations. Since many hydrocarbon producing formations, such as sands and limestones, are in this latter category, this is a matter of major concern. Conventional coring bits typically utilize relatively large, discrete, cutters which serve to cut the formation efficiently. Such conventional bits include cutters distributed from proximate the inner gage of the bit, along the bit contour, to the outer gage. The bits typically include both inner and outer gage cutting sections formed of vertical rows of surface-set natural diamonds. Additionally, these core bits typically provide for discharging drilling fluid adjacent the core to lubricate these inner gage cutting portions.
The disadvantages of these conventional systems are substantial. For example, the abrasive cutting of the surface-set diamonds on the inner gage of the bit puts substantial strain on the relatively fragile core produced from a soft to medium hard formation, promoting breakage of the core. Such core breakage, in addition to being damaging to the core, and thereby to its value as a formation indicator, will also frequently cause core jamming in the core barrel, leading to a premature and undesired end to the coring operation. Additionally, and critically, the fluid discharge required for lubrication of the gage cutting section promotes fluid invasion of a fractured or permeable core, again promoting deterioration of the core, both structurally and, most importantly, as a formation sample. This fluid invasion of the core is a major problem and may be especially severe with particular types of drilling fluids.
Accordingly, the present invention provides a new method and apparatus for coring a formation whereby the exposure of the core to drilling fluids may be minimized, to prevent the core from fluid invasion, and whereby the core may be immediately inserted into the core shoe after cutting to minimize mechanical strains and fluid exposure on the core.